How to make a rectangular light in mental ray?

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Old 08 August 2011   #16
There's nothing like picture examples to really demonstrate the problem. Thanks guy for explaining this to me (and everyone). I'll be sure to use the sky portal but also be aware that it won't be that fantastic.

Jeff, in your car shoot example, would 20 sky portals abutting each other reduce the problem a little?

Would that also smash the render time?
Old 08 August 2011   #17
Originally Posted by NSF: Jeff, in your car shoot example, would 20 sky portals abutting each other reduce the problem a little? Would that also smash the render time?
A large group of area lights would indeed take much longer to render than one. I have used some custom light shaders in the past but they all have some kind of drawback/limitations so it's just something that needs to be addressed correctly.

I hesitate to call this an error since I believe the center point emission is by design. I think it has something to do with assuring IES accuracy (I think) and may be necessary for the lighting analysis accuracy too (again, I think).

Geometry lights may be the easiest solution for those of us that aren't concerned with center point accuracy on area lights since mental ray supports geometry lights anyway. At any rate I sure hope we will see some kind of solution to this one soon.
Jeff Patton
My Website
Old 08 August 2011   #18
Hi Miked08,

my english is not good, but i hope that can help. The benefit of Portallight for me are two things.

- cutoff treshold ( spread angle of Light emitting)
- Shadow Ray extension ( as wich distance is Raytraceshadow active )

ceiling Light:
At the Scene i use two portal Light. One light for the Window and one light for the star ceiling, because i want only one light for all stars/spots. The ceiling light position is in the ceiling, and i use Shadow Ray extension to eliminate the Shadow from the ceiling, because my Light is in the ceiling. If my light outside, i become a Shadow on the ceiling, and that look not realistic.

Demofile 3dsmax2012:

have fun!

hot chip

Last edited by thorsten hartmann : 08 August 2011 at 03:27 PM.
Old 08 August 2011   #19
Just out of curiosity, is this the same for Iray or does the light actually show up as an even square?

It would be nice if setting the light to Uniform Diffuse would mimic the perfect square results in MR.
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Last edited by Decency : 08 August 2011 at 08:27 PM.
Old 08 August 2011   #20
Just mental ray. Same test scene rendered with iray below:
Jeff Patton
My Website

Last edited by JeffPatton : 08 August 2011 at 07:17 PM.
Old 08 August 2011   #21
Wow, well that pushes me more towards the opinion that MR is indeed incorrect, and that Vray and Iray are more accurate.
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Old 08 August 2011   #22
for me look the mr portal light, the same as iRay rectangle light. But all the lights generate not a harmonic light emitting.
Old 08 August 2011   #23
Hi thorsten, thanks for putting the files and images up but unfortunately I can't download the .zip file and can only see half of the images. The place where you are hosting the files is really slow and keeps cutting out mid download.
Old 08 August 2011   #24
hi mike,

i can download the file without problems, it comes directly from my website.
Old 08 August 2011   #25
Ah yes, it's working now, many thanks for the setup, I'll get studying it!
Old 11 November 2011   #26
Originally Posted by thorsten hartmann: Hi

The Trick of Jeff´s PortalLight work here very good. I use the original mia_portal_light Shader, and have more options to optimize the outcoming light.

@Miked08: mental ray can Mesh-Light/Object-Light, but the Autodesk Coder can not, or want not programming the features. I think they can not, because we tell it for years.

hot chip

Hi Thorsten, I couldn't get the mia_portal_light Shader to work. If i enable it in the kight shader dialog, the light emits dark [no light] do you do any setup on the portal light shader map. Also where did you get the architectural environment portal you have in the sample files. {Is it some other hidden string]

Old 01 January 2012   #27
Quote from Master Zap:

'Basically, the distribution of light from a flat plane can be calculated properly by taking samples and taking the traditional distance-squared for *each sample*. While this _works_, it yeilds a totally horriblke amount of noise that you need to turn up the samples to high heaven to get rid of.

The portal is designed to be _smooth_, and approximates the "sum of an infinite set of distance squared to every point of the light" with an approximation, such that it should have no noise created by sampling (outside of the inevitable noise caused by the cosine of the angle to each sample).

This algorithm is a bit of thumb-and-forefinger and is not totally accurate, and especially on a long-thin light it apparently breaks down a little bit. Note that this algorithm is _only_ actually applied to light close to the lightsource itself, anything further smoothly interpolates towards a traditional distance-square based calculation (since the noise problem goes away at any significant distance).

So the light may be a little bit off in the extreme nearground, but it is specificaly designed for smoothness. At any distance (e.g. at a distance of about the longest axis of the light), it is accurate.

That's the idea anyway

Old 01 January 2012   #28
Well vray seems to have no trouble doing it correcly without the mentioned horrible amount of noise, so...
Old 01 January 2013   #29
Originally Posted by JeffPatton: Just mental ray. Same test scene rendered with iray below:

I tested on mental ray maya 2012,and found that area light is ok.

It seems strange.
Old 01 January 2013   #30
Originally Posted by david376: I tested on mental ray maya 2012,and found that area light is ok.

It seems strange.
That's because it's a 3ds Max "feature".
Jeff Patton
My Website
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